The smaller ribosomal subunit is responsible for binding messenger RNA, for recognizing initiation sites on mRNA with the participation of initiation factors and fmet-tRNA, for sequentially binding correctly charged tRNA's according to each triplet codon on the mRNA, and for associating with the large subunit during the ribosome cycle. In order to understand events occurring during protein synthesis, at the molecular level, it will be necessary to know the three dimensional distribution of small subunit proteins and factors on the ribosome. The positions of 30S proteins and initiation factors will be determined using specific antibodies. Small subunits reacted with antibodies against specific 30S proteins or initiation factors will be observed in the electron microscope and the points of antibody attachment located. Unique three dimensional sites for each protein will be ascertained by comparing images of small subunits (with attached antibodies) at different orientations. In the case of elongated proteins, such as protein S4, antibodies directed against parts of the primary sequence will be used to locate specific regions of the molecule.